Petroleum production methods currently are utilizing high pressure nitrogen injection to maintain well head pressure for enhanced oil and gas recovery. As nitrogen is injected, the natural gas from the well containing methane and associated hydrocarbon liquids also contains nitrogen which increases in amount over the life of the nitrogen injection project. There are also gas fields which naturally contain high levels of nitrogen at an essentially constant or fixed composition. For this reason, natural gas containing nitrogen must be separated to reject the nitrogen and form purified natural gas feedstocks suitable for utilization as fuel or chemical feedstocks.
Several processes have either been proposed or commercially utilized to reject nitrogen from natural gas streams containing fixed or variable nitrogen contents.
U.S. Pat. No. 3,516,262 discloses a process, for separating methane from mixtures of methane and nitrogen. In the process, a mixture of methane and nitrogen is separated in an initial separation stage to produce a cold nitrogen vapor and a methane-rich liquid. The methane-rich liquid is fed to a fractionating column in heat exchange relation with the initial separation stage. The cold nitrogen from the initial separation stage is recycled at a reduced temperature in heat exchange relationship along the upper portion of the fractionating column and the inital separation zone. The liquid-vapor mixture in the lower section of the initial separation zone provides heat and reboiler duty to the lower portion of the fractionating column. The initial separation zone and fractionating column are operated under conditions to effect a "differential" distillation in the column.
U.S. Pat. No. 3,683,634 discloses a process which especially suitable for natural gas containing a low concentration, i.e. 1 mole percent or less, of carbon dioxide from which the nitrogen and part of the methane are to be recovered in substantially pure form by separation in a two-stage rectification zone. In accordance with the process, a portion of a pressurized gas mixture which contains one or more components that are precipitated in solid form during cool-down, is separated in a prefractionation zone into a fraction essentially free of such components and a fraction containing a higher concentration of such components.
U.S. Pat. No. 3,797,261 discloses the separation of natural gas containing nitrogen into a low-nitrogen fraction and a high nitrogen fraction by distillation in a single distillation column by expanding the high-nitrogen fraction with the performance of work and using the resulting refrigeration to condense vapor in the upper section of the column while additional reflux is provided by vaporizing a recycle medium in heat exchange relation with vapor in the column. The high-nitrogen mixture, having been expanded, is exhausted at atmospheric pressure.
U.S. Pat. No. 4,158,556 discloses a process for separating nitrogen from natural gas hydrocarbons by using a column with a single fractionation zone operating at feed pressure, work expanding the overhead nitrogen to produce a cold nitrogen stream, utilizing the resultant cold nitrogen stream in continuous indirect heat exchange with column vapors to provide reflux, and generating additional reflux by boiling liquid bottoms in continuous indirect heat exchange with the lower section of the column.
U.S. Pat. No. 4,411,677 discloses a process for rejecting nitrogen from a natural gas feed containing nitrogen over a broad range of compositions, under elevated pressure using a single distillation column, and a closed loop methane heat pump which reboils and refluxes the column. An intermediate reflux condenser is cooled by both the heat pump and overhead nitrogen stream of the column. A mixed cryogenic refrigerant can be used in the heat pump as an alternative to the methane heat pump medium. The process provides a high methane recovery over the entire feed range, and provides a pressurized nitrogen product stream that can be used for recycling and reinjection to an oil or gas well to improve well head pressure.
U.S. Pat. No. 4,415,345 discloses a process for separating by rectification, low concentration nitrogen from natural gases having a gradually increasing nitrogen concentration which employs a nitrogen heat pump cycle to generate the necessary liquid reflux for a fractionation column. The process disclosed in the patent is compatible for use with both single column and double column process arrangements.
U.S. Pat. No. 4,455,158 discloses a process for cooling a multicomponent gas stream containing variable amounts of the components by passing the gas stream through a heat exchange relationship with a fluid coolant stream so that carry-up of the condensed phase is maintained without the condensed phase backmixing over the compositional range of the multicomponent gas stream. The gas stream is cooled by passing it through a cold-end up heat exchanger having a serpentine pathway for the multicomponent gas stream comprising a series of horizontal passes separated by horizontal dividers and alternatingly connected by turnaround passes at each end, the cross-sectional area of at least one horizontal pass nearer the cold end being less than the cross-sectional area of a horizontal pass nearer the warm-end. The process is especially applicable to cooling a natural gas stream having a variable nitrogen content in a nitrogen rejection process.
U.S. Pat. No. 4,504,295 discloses a process for the recovery of methane, nitrogen and natural gas liquids from a natural gas feed stream wherein the recovery can be made by the integration of a nitrogen rejection stage including a heat pump driven distillation column and a natural gas liquids recovery stage. Nitrogen can be rejected over a wide range of nitrogen concentrations in the feed stream.